(180a) Selective Glucose Isomerization to Fructose Using a Heterogeneous Immobilized Tertiary Amine with Tuned Molecular Design
Biomass conversion has the potential to transform our ability to produce chemicals from renewable resources. Indeed, many biomass derived compounds originate from highly functionalized molecules such as cellulose that can be hydrolyzed to produce glucose. The key challenge is converting these compounds into useful chemicals and intermediates such as 5-hydroxymethylfurfural (HMF). A key step in this process is the isomerization of glucose to fructose. Commercial processes use enzymes to convert glucose to fructose, but this process requires highly controlled conditions. Recent work identified homogeneous tertiary amines as promising catalysts with similar selectivities as Sn-Beta. Translating the interesting catalytic behavior of these tertiary amines to heterogeneous catalysts has the potential to overcome current limitations for selective isomerization of glucose to fructose. In this work, we will discuss our recent efforts to design heterogeneous catalysts with immobilized tertiary amines. We have discovered that tuning the structure of the amine can have a significant impact on the catalytic activity. Initial work determined that catalytic activity is strongly affected by the alkyl linker length, the substituents on the amine, and the surface loading. For heterogeneous amines, we have shown that it is important to mitigate amine-silane interactions through using a short alkyl linker, bulky substituents, and high loadings. Whereas these features increase catalytic activity, these catalyst had limited stability as the organosilane would leach from the surface. Indeed, catalysts containing a propyl (C3-SBA) or methyl (C1-SBA) linker deactivated through leaching of the organosilane, reducing the loading of the fresh catalyst after one use, as shown in the Figure. We have overcome this challenge through introducing a hydrophobic phenyl group in the organosilane backbone (Ph-SBA). After each cycle, the Ph-SBA loading remains constant and the catalyst remains active. Overall, this work demonstrates that the base-catalyzed isomerization of glucose to fructose can be selective.